Where did Toothless crash?
First of all: Hiccup’s drawing is not very reliable. His dotted arc would not make any sense unless Toothless was able to utilize his wings to turn. But:
- That wouldn’t benefit Toothless in any way when he is falling, and
- His wings are quite restricted due to the bolas.
So, we can forget about the arc. We know from the movie (11:37 - 12:00) that Hiccup marks the most upper-right X, turns about 45 degrees to the left, and continues walking until he notices Toothless’s crash path, directing around 45 degrees to his left. This should help us.
I am here assuming that Toothless was flying at an angle of around 100 degrees to the bolas’ trajectory, such that when the bolas’ and Toothless’s momentum get combined in the collision, the direction of the fall should be as shown in the picture above due to the law of conservation of momentum. We can then place Hiccup’s path according to this direction, with the upper-right X as a reference point, and see that the green mark should be approximately where he crashed.
But it also raises a question: Why was Hiccup walking from the seaside, where there are no markers?